Idling, or an idling power producing situation, is a condition in which a wind turbine is not producing electrical power to a utility grid to which it is connected. A case in which the turbine might be idling is where the wind speed is below wind speeds in which the turbine is adapted to operate. Another case in which the turbine might be idling is where the wind speed is above wind speeds in which the turbine is adapted to operate, i.e. above a so called cut-out wind speed, which can be 25 m/s. Idling might also be caused also by faults in the system, e.g. in the turbine or in the grid, or by constraints coming from a grid operator.
A problem that can occur is tower oscillation during idling, which can be caused e.g. by high winds. Offshore, the situation can be worsened by sea waves hitting the tower bottom or the tower foundation, causing additional promotion of oscillation. The oscillation involves an oscillatory motion of the tower and nacelle, in a longitudinal (parallel with the rotor axis) and/or a transversal (lateral) direction. This can lead to fatigue damage to the wind turbine structures.